Abstract

Persistent volcanic activity is thought to be linked to degassing, but volatile transport at depth cannot be observed directly. Instead, we rely on indirect constraints such as CO2-H2O concentrations in melt inclusions trapped at different depth, but this data is rarely straight-forward to interpret. In this study, we develop a multiscale model of conduit flow during passive degassing to identify how flow behavior in the conduit is reflected in melt-inclusion data and surface gas flux. During the approximately steady flow likely characteristic of passive-degassing episodes, variability in degassing arises primarily from two processes, the mixing of volatile-poor and volatile-rich magma and variations in CO2 influx from depth. To quantify how conduit-flow conditions alter mixing efficiency, we first model bidirectional flow in a conduit segment at the scale of tens of meters while fully resolving the ascent dynamics of intermediate-size bubbles at the scale of centimeters. We focus specifically on intermediate-size bubbles, because these are small enough not to generate explosive behavior, but large enough to alter the degree of magma mixing. We then use a system-scale volatile-concentration model to evaluate the joint effect of magma mixing and CO2 influx on volatile concentrations profiles against observations for Stromboli and Mount Erebus. We find that the two processes have distinct observational signatures, suggesting that tracking them jointly could help identify changes in conduit flow and advance our understanding of eruptive regimes.